The science of supersonic: What is supersonic flight, why did Concorde end and will it come back?

Supersonic transport (SST) was a dream that became reality, and then became a dream again. Cold War competition meant the 1950s and 60s saw Russia, the US, Britain and France compete to translate supersonic technology into commercial flight. The latter two eventually came out on top, in the long, beaked-nose shape of the Concorde. Flights from London to New York suddenly took under 3.5 hours; the future of air travel was here.

And then it went away. Since 2003, there has been no supersonic transport. The technology has become, once again, the domain of military and experimental aircraft. So what happened? Why was supersonic transport unsustainable, and could it be making a comeback?

Watch: This plane could fly from London to New York in 20 minutes (Wochit)

What is supersonic flight?

A supersonic aircraft is one that is, simply put, able to fly faster than the speed of sound – roughly 1,235 km/h (767 mph). By far the most common use of this technology is in the military, with fighter jets often designed to perform at supersonic speeds.

Supersonic flight is one of four speeds of flight, the full list being subsonic, transonic, supersonic and hypersonic. When talking about aircraft speeds in ratio to the speed of sound, the Mach system is used. Mach 1 is the speed of sound. Lower than Mach 1 is subsonic, higher than Mach 1 is supersonic.

When you get above Mach 5 (five times faster than the speed of sound) you reach hypersonic speeds. Transonic is roughly around Mach 1, and at this speed the aircraft may find itself with a range of velocities of airflow.

Supersonic transport (SST) for the public centred around two aircraft: the Anglo-French Concorde and the Russian Tupolev Tu-144. Both had their first flight towards the end of the 1960s and were introduced into commercial travel towards the end of the 1970s. Following crashes during testing, the Russian aircraft only managed 55 passenger flights before it was grounded. That made Concorde the sole SST aircraft until its retirement in 2003.

How does supersonic flight work?

For supersonic flight to work, the aircraft needs to move and sustain speeds faster than Mach 1 (the speed of sound).

The design of a supersonic aircraft is, as you’d expect, quite complicated. In broad terms, the aircraft needs to deal with enormous amounts of aerodynamic drag and therefore requires a streamlined shape; a way to deal with large amounts of heat caused by friction; and a powerful engine to provide a lot of thrust.

For example, Concorde was able to manage cruising speeds of just over Mach 2. It used the Olympus 593 turbojet engine, along with afterburners to inject extra fuel into the jet pipe, and increase thrust during takeoff. An aluminum alloy was used to sustain heat caused by friction – keeping things together up to around 127 °C.

In terms of shape, the Concorde’s iconic design encompassed a long, thin body, as well as a “drooping nose” that could be lowered for takeoff and landing – which required a high angle of attack, meaning a steep liftoff or touchdown – and raised to increase streamlining during supersonic flight. The wings were a delta ogival (triangular and curved) shape – tailored for airflow at high speeds.

Why did supersonic travel come to an end?

By the start of the 21st century, only Concorde remained as a commercial supersonic aircraft. This would soon come to an end – with the aircraft’s last flight in 2003.

Why did SST peter out? In July 2000, a Concorde aircraft crashed soon after takeoff from Paris-Charles de Gaulle, killing all 109 people on the flight as well as four people on the ground. While this was the only fatal accident to happen to Concorde, it meant the line was grounded until November 2001. By then, the industry had financially fallen in the wake of the 9/11 attacks.

Even without these events, SST faced a heap of challenges. The sheer scale of a “sonic boom”, when the aircraft breached Mach 1, was enough to break people’s windows on ground level. This meant supersonic flight was only feasible over oceans, limiting demand for the service. There were also serious environmental concerns, centred on the damage an SST fleet – travelling at very high altitudes – could pose to the ozone layer. Low fuel efficiency also limited the scope for travel, meaning it could only really be used for transatlantic travel.

There’s also the fact transport on Concorde was extremely expensive, with round-trip tickets reaching highs of around $12,000 (£9,000).

Add all of this up, and Concorde didn’t make economic sense. The tragic accident of 2000 might have precipitated the aircraft’s downward spiral, but the ultimate reason SST fell apart was because it didn’t make enough money to sustain its high costs.

While there are suggestions firms such as Boom Supersonic could signal a return to SST, those companies will need to contend with the same questions that plagued Concorde in its final years – how do you scale supersonic travel to keep it in the air?

Supersonic flights faster than Concorde could be launching sooner than you think

Commercial supersonic flight has been grounded since Concorde made its final trip, but new reports suggest super-fast air travel may be making a comeback.

According to The Market Mogul, and originally picked up by Business Insider, aircraft firm Boom Supersonic has received 76 pre-orders for its upcoming Boom XB-1. The supersonic plane is said to reach speeds of 2,715 km/h (1,687 mph) – substantially faster than the Concorde’s 2,179 km/h (1,354 mph) – and could carry 55 passengers between London and New York in three hours and 15 minutes.

The XB-1 is pitched by its creators as being faster, smaller and quieter than Concorde, apparently designed not to create the latter’s loud “sonic boom”, which disrupted people in its flight path. Boom Supersonic will also need to prove that the new aircraft is safer – in July 2000 an Air France Concorde killed 113 people after it crashed, leading to the decline and eventual demise of Concorde.

Boom Supersonic has said it aims to have a smaller version of the XB-1 to test by the end of 2018, with the full-sized aircraft operational by 2025. A big barrier to all this is the fact supersonic air travel is currently illegal over the United States, making that London to New York trip much harder to pull off.

If that law were to be overturned it could signal a return for commercial supersonic flight. Here’s an explainer on what supersonic travel is, and why it came to an end in 2003.